Apparatus and method of sound attenuation in a system employing a VSD and a quarter-wave resonator

ABSTRACT

A resonator in a closed fluid system includes a body having a passageway in communication with a closed fluid system wherein a piston is movable within the passageway. Each position of the piston within the passageway defines a noise attenuation frequency corresponding to a noise frequency generated by the closed fluid system. A device providing a pressurized fluid from the closed fluid system selectively moves the piston within the passageway to vary the noise attenuation frequency.

FIELD OF THE INVENTION

The present invention relates generally to a method of operation andapparatus for noise attenuation of closed fluid systems, and moreparticularly, to a method of operation and apparatus for adjustablenoise attenuation of closed HVAC&R systems having positive displacementcompressors with variable speed drives.

BACKGROUND OF THE INVENTION

Heating, ventilation, air conditioning or refrigeration (HVAC&R) systemstypically maintain temperature control in a structure by circulating afluid within coiled tubes such that passing another fluid over the tubeseffects a transfer of thermal energy between the two fluids. A primarycomponent in such a system is a positive displacement compressor, whichreceives a cool, low pressure gas and by virtue of a compression device,exhausts a hot, high pressure gas. One type of positive displacementcompressor is a screw compressor, which generally includes twocylindrical rotors mounted on separate shafts inside a hollow,double-barreled casing. The side-walls of the compressor casingtypically form two parallel, overlapping cylinders which house therotors side-by-side. Screw compressor rotors typically have helicallyextending lobes and grooves on their outer surfaces forming a largethread on the circumference of the rotor. During operation, the threadsof the rotors mesh together, with the lobes on one rotor meshing withthe corresponding grooves on the other rotor to form a series of gapsbetween the rotors. These gaps form a continuous compression chamberthat continuously reduces in volume as the rotors turn to compress thegas and which communicates with the compressor inlet opening, or “port,”at one end of the casing and with a discharge port at the opposite endof the casing.

These rotors rotate at high rates of speed, and multiple sets of rotors(compressors) may be configured to work together to further increase theamount of gas that can be circulated in the system, thereby increasingthe operating capacity of a system. While the rotors provide acontinuous pumping action, each set of rotors (compressor) producespressure pulses as the pressurized fluid is discharged at the dischargeport. These discharge pressure pulsations act as significant sources ofaudible sound within the system. In addition, mechanical noise isgenerated by the meshing and unmeshing of the rotors which is the resultof backlash and movement between the rotors.

To eliminate or minimize the undesirable sound, noise attenuationdevices or systems can be used. Noise attenuation devices includereflective and absorptive mufflers. Absorptive mufflers are mosteffective when the frequencies of the sound are greater than 500 Hz.Since the fundament frequencies for a positive displacement compressor,such as a screw compressor, are typically less than 500 Hz, absorptivemufflers are less effective than other types of mufflers.

However, the pressure pulse frequency of screw compressors arepredictable, that is, the pressure pulse frequency is a function of thenumber of lobes of the male rotor multiplied by its rotational speed.For example, a screw compressor having a 5 lobed male rotor, whichrotates at 3,600 RPM generates a frequency of 5×3, 600/60 or 300 Hz.Therefore, for a compressor, such as a screw compressor, operating at afixed speed, a reflective muffler, such as a side branch resonator,which is also referred to as a quarter wave resonator, may be configuredto attenuate noise at a specific frequency. The side branch resonatortypically comprises a component that forms a tee “T” adjacent acompressor discharge, with the tee being closed at the end opposite thecompressor discharge. The length of the side branch resonator isconfigured or “tuned” to be one-fourth of one wavelength of the soundproduced by the compressor to achieve the desired noise attenuation.

Unfortunately, even with the compressor operating at a fixed speed,there are variations in ambient conditions, such as pressure andtemperature of the refrigerant gas used in the system. Variations in theambient conditions can likewise vary the frequency of the noise producedby the system, as such ambient condition variations can likewise changethe acoustic velocity of the refrigerant gas. The term acoustic velocityis defined as the velocity that sound travels through the refrigerantgas for a given set of ambient conditions. Thus, it is generally notpossible to attenuate the noise generated by a fixed speed compressorwith a single side branch or quarter-wave resonator. Further, forreasons of increased compressor efficiency, it is often desirable toemploy variable speed drives to power the compressor motors, resultingin compressors running at variable speeds.

What is needed is a cost-effective, efficient and easily implementedmethod or apparatus for compressor noise attenuation that may be usedwith variable speed compressors.

SUMMARY OF THE INVENTION

The present invention relates to a resonator arrangement for a closedfluid system including a body having a passageway in fluid communicationwith a closed fluid system. A piston is movable within the passagewaywherein a position of the piston within the passageway defines a noiseattenuation frequency for the closed fluid system. A control arrangementselectively positions the piston within the passageway to generate anoise attenuation frequency corresponding to a noise frequency generatedby the closed fluid system.

The present invention further relates to a variable noise attenuationdevice for use with an HVAC&R fluid system including a body having apassageway in fluid communication with an HVAC&R fluid system. A pistonis movable within the passageway wherein a position of the piston withinthe passageway defines a noise attenuation frequency for the HVAC&Rfluid system. A proportional valve selectively provides a pressurizedfluid from the HVAC&R fluid system to the piston to selectively positionthe piston within the passageway to generate a noise attenuationfrequency corresponding to a noise frequency generated by the HVAC&Rfluid system.

The present invention yet further relates to a method for attenuatingnoise in a closed fluid system having the steps of providing a bodyhaving a passageway in fluid communication with a closed fluid system;providing a piston movable within the passageway wherein a position ofthe piston within the passageway defines a noise attenuation frequencyfor the closed fluid system; providing a control arrangement forproviding a pressurized fluid to the piston; and positioning selectivelythe piston within the passageway to generate a noise attenuationfrequency corresponding to a noise frequency generated by the closedfluid system.

An advantage of the present invention is that it attenuates noise ofvariable frequency.

A further advantage of the present invention is that it is inexpensiveto manufacture.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a variable speed HVAC&R system having avariable frequency resonator of the present invention.

FIG. 2 is an enlarged partial schematic of the HVAC&R system includingan enlarged elevation view of the variable frequency resonator of thepresent invention.

FIG. 3 is an enlarged partial schematic of the HVAC&R system includingan enlarged elevation view of an embodiment of the variable frequencyresonator of the present invention.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates generally a HVAC&R system 10 incorporating thepresent invention. An AC power source 12 supplies AC power to a variablespeed drive (VSD) 14, which in turn, supplies AC power to a motor 18 fordriving a compressor 20. A control panel 16 controls both the outputfrequency and the voltage from VSD 14 that is supplied to motor 18,which selectively varies both the torque and speed of motor 18, andlikewise varies the speed of compressor 20. Control panel 16 includes ananalog to digital (A/D) converter, a microprocessor, a non-volatilememory, and an interface board to control operation of the refrigerationsystem 10. The control panel 16 can also be used to control theoperation of the VSD 14, the monitor 18 and the compressor 20.Compressor 20 receives refrigerant gas at its inlet and dischargescompressed refrigerant gas from its outlet through discharge line 30.Optionally, other compressors may operate in parallel with compressor 20to increase the capacity of the system 10. These compressors aretypically positive displacement compressors, such as screw,reciprocating or scroll, having a wide range of cooling capacity, butmay also include other compressor constructions.

After refrigerant gas is discharged by compressor 20 through dischargeline 30 toward condenser 22, the refrigerant gas is directed past avariable resonator 26, such as a quarter wave resonator. Variableresonator 26 is continuously tuned, as will be described in furtherdetail below, to attenuate both the pressure pulses and the mechanicalnoise generated by the meshing and unmeshing of the rotors that is theresult of backlash and movement between the rotors. The variableresonator 26 can attenuate both the pressure pulses and the mechanicalrotor noise, since each occurs at substantially the same frequency. Oncethe refrigerant gas passes variable resonator 26, it is seriallydirected to condenser 22 and then to evaporator 24 where the refrigerantgas is placed in a non-mixing heat exchange relationship with fluidsoutside the system 10 to maintain temperature control in a structurebefore returning the refrigerant gas to the compressor 20 to repeat thecycle. The conventional HVAC&R system 10 includes many other featuresthat are not shown in FIG. 1. These features have been purposely omittedto simplify the drawing for ease of illustration.

Referring to FIG. 2, variable resonator 26 includes a body 38, such as atube or cylinder, which defines a passageway 39. A stop 48 having anaperture 49 is secured to one end of body 38, and a cap 40 is secured tothe opposite end of body 38. The end of body 38 having the stop 48 isconnected to discharge line 30 so that passageway 39 of body 38 is influid communication with the refrigerant gas flowing through dischargeline 30. A piston 42 is inserted inside of passageway 39 that isslidable or movable within passageway 39, the piston 42 further includesa seal 44, such as an O-ring, to provide a substantially fluid tightseal between piston 42 and an inside surface 41 of body 38 as piston 42slides along passageway 39. A resilient device 46, such as a spring, isinterposed between piston 42 and cap 40 to urge piston 42 into movementalong passageway 39 in a direction away from cap 40, the travel of thepiston 42 in this direction being limited by stop 48. Thus, the travelof the piston 42 along passageway 39 is limited by the stop 48, in onedirection, and by the cap 40, minus the thickness of the compressedresilient device 46, in the other direction.

It is desirable to control the position of the piston 42 within thepassageway 39, in which a variable resonator length 50 is defined by thedistance between the lower surface of piston 42 and the lower surface ofstop 48. The variable resonator length 50 of resonator 26, which is aside branch resonator, operates in accordance with the followingequation stated symbolically asF=C/(4×1)  [1]

Wherein F is the resonant frequency of the resonator 26; C is the sonicspeed or acoustic velocity of the fluid traveling through the resonator26; and 1 is the resonator length 50. Thus, the variable resonatorlength 50 preferably corresponds to one quarter of a wavelength, such asthe wavelengths generated during the operation of the compressor 20,which can be attenuated by the resonator 26. While the single positionof a fixed length resonator may effectively attenuate noise of aspecific wavelength corresponding to a compressor operating at a fixedspeed and a specific set of ambient conditions, such unchangingoperating conditions are unrealistic. In other words, changing ambientconditions, such as temperature, also changes the density of therefrigerant, which changes the sonic speed C of the refrigerant, whichchanges the resonator length required to attenuate the noise generatedby the compressor as discussed in equation [1]. A fixed length resonatorcannot accommodate these changing conditions. Additionally, thecompressor of the present invention operates at variable speeds, which,in turn, results in variable frequencies. Thus, selective control of theposition of the piston 42 within the passageway 39 of the resonator 26is required for consistent, noise attenuation.

To achieve positional control of the piston 42 within the passageway 39,a valve 28, such as a proportional valve, is controlled by the controlpanel 16 to selectively provide pressurized refrigerant gas from apressure line 36 through cap 40. To increase the amount of pressure inthe pressure line 36, the control panel 16 actuates the valve 28 tobleed pressurized refrigerant gas from a high pressure line 34, which isin fluid communication with the compressor discharge 30, to provide tothe pressure line 36. Conversely, to decrease the amount of pressure inthe pressure line 36, the control panel 16 actuates the valve 28 tobleed pressurized refrigerant gas from the pressure line 36 to provideto the low pressure line 32, which may be connected to a port in thecompressor 20, or to the suction line at the inlet of the compressor 20.Alternately, the low pressure line 32 may be connected to a separatecontainer (not shown) that is maintained at an even lower pressure thanthe suction line at the compressor inlet to provide an even wider rangeof pressure differential, if desired. In other words, the amount ofpressure in the pressure line 36 can range anywhere from the dischargepressure of the high pressure line 34 to the lower pressure contained inthe low pressure line 32 to effect movement of the piston 42 along thepassageway 39 of the resonator body 38.

To effect movement of the piston 42 within the passageway 39, the netforce that is applied to the surface of the piston 42, which is facingthe cap 40 must be sufficiently different from the net force that isapplied to the opposite surface of the piston 42 which faces theaperture 49. There must be a sufficient difference in forces acting onthe opposed surfaces of the piston 42 to overcome any combination offrictional forces between the seal 44 and the inside surface of body 38,the weight of the piston 42 (if the piston moves vertically) and inertiaof the piston 42. The net force that is applied to the surface of thepiston 42 facing the cap 40 is applied in two parts. The first part ofthe net force is the pressure of the refrigerant gas in the pressureline 36 (P_(LINE)) multiplied by the surface area of the piston 42(A_(SURFACE)) facing the cap 40. The second part of the net force is theforce applied by the resilient device 46, which is the product of thespring constant (k) of the spring multiplied by the distance (d) thespring is compressed. The sum of the forces applied to the surface ofpiston 42 facing cap 40 (F_(CAP)) is stated symbolically asF _(CAP)=(P _(LINE) ×A _(SURFACE))+(k×d)  [2]

The net force that is applied to the surface of the piston 42 facing thestop 48 is applied in a single part. This single net force part is thepressure of the refrigerant gas in the discharge line 36 (P_(DISCHARGE))multiplied by the surface area of the piston 42 (A_(SURFACE)) facing thestop 48. The force applied to the surface of the piston 42 facing thestop 48 (F_(STOP)) is stated symbolically asF _(STOP) =P _(DISCHARGE) ×A _(SURFACE)  [3]

Depending upon the configuration of the HVAC&R system, the P_(DISCHARGE)may or may not vary. For example, if the system uses a water-cooledcondenser, P_(DISCHARGE) is considered substantially constant over therange of operating speeds of the compressor. In contrast, if anair-cooled condenser is used, the P_(DISCHARGE) fluctuates, thefluctuation being either variable or substantially constant, dependingupon the operating conditions. However, over an extreme range ofoperating conditions using the air-cooled condenser, it has beendetermined that the magnitude of P_(DISCHARGE) will vary no more thanabout one percent of the variable resonator length 50. Thus, the effectof P_(DISCHARGE) fluctuation can typically be disregarded, while it isthe sonic speed of the refrigerant that must be monitored.

Therefore, when there is a sufficient difference between the F_(CAP) andthe F_(STOP) forces, the piston 42 is urged to move along the passageway39 in a direction away from the larger force. In other words, the piston42 will continue to move in this direction until the forces aresubstantially equal, or the piston 42 has reached the end of itspossible travel, that is, abutting the stop 48 or abutting the cap 40,minus the thickness of the compressed resilient device 46. When thepiston 42 abuts stop 48, the variable resonator length 50 is at itsshortest length and corresponds to the shortest one-quarter wavelengthposition, and therefore, the shortest wavelength, or highest frequency,that the resonator 26 can attenuate. Conversely, when the piston 42abuts the cap 40, minus the thickness of the compressed resilient device46, the variable resonator length 50 is at its maximum length, andcorresponds to the longest one-quarter wavelength position, andtherefore, the longest wavelength, or lowest frequency, that theresonator 26 can attenuate. It is appreciated by those skilled in theart that there exists a reciprocal relationship between frequency andwavelength.

The position of the piston 42 is controlled by the control panel 16using a control algorithm. Based upon application of equations [1]-[3],the control algorithm determines the desired resonator length 50, thatis, the desired position of the piston 42 within the passageway 39, byreading the VSD frequency output that is supplied to the compressormotor 18, and comparing the desired position of the piston 42 to theposition of the piston 42 in the passageway 39. The control algorithmmay employ a look-up table that accounts for changes in ambientconditions, such as temperature and pressure, which likewise can changethe acoustic velocity of the gas, and the desired position of the piston42 within the passageway 39. Further, the control algorithm may furtheremploy the look-up table or calculate the relationship between thespring length (d in equation [2]) and the piston 42 position withoutneeding to measure the piston 42 position. Alternatively, the positionof the piston 42 in the passageway 39 may be determined by a sensor, arheostat, or any other suitable mechanical or electrical device thatprovides such positional information to the control panel 16. Inresponse to comparing the desired position of the piston 42 to itsactual position within the passageway 39, the control panel 16 sends acontrol signal(s) to the valve 28. If the control algorithm determinesthat the variable resonator length 50 should be decreased, the controlsignal from the control panel 16 causes the valve 28 to actuate to bleedpressurized refrigerant from the high pressure line 34 to the pressureline 36, thereby increasing the pressure in the pressure line 36. Thecombined forces, generated from the pressure in the pressure line 36 andthe compressed resilient device 46, are applied to the piston 42, whichcollectively define force F_(CAP) as previously discussed in equation[2]. The opposing force that is applied to the piston 42 defines forceF_(STOP) as previously discussed in equation [3]. When the F_(CAP) forcesufficiently exceeds the F_(STOP) force, the piston 42 slides toward thestop 48, thereby decreasing the variable resonator length 50.

Conversely, if the algorithm determines that the resonator length 50should be increased, the control signal from the control panel 16 causesthe valve 28 to actuate to bleed pressurized refrigerant from thepressure line 36 to the low pressure line 32, thereby decreasing thepressure in the pressure line 36. The combined forces generated from thepressure in the adjustable pressure line 36 and the compressed resilientdevice 46 are applied to the piston 42, which collectively define forceF_(CAP) as previously discussed in equation [2]. The opposing force thatis applied to the piston 42 defines force F_(STOP) as previouslydiscussed in equation [3]. When the F_(STOP) force sufficiently exceedsthe F_(CAP) force, the piston 42 slides toward the cap 40, therebyincreasing the variable resonator length 50.

Although a preferred embodiment of the resonator 26 discloses the valve28 controlling the position of the piston 42 by selectively providingpressurized refrigerant gas between lines 32, 34 and 36 using therefrigerant gas from the HVAC&R system 10, it is appreciated thatpressurized refrigerant from a source that is independent from theHVAC&R system 10 may also be used. Additionally, other pressurizedfluids each from independent sources of pressurized fluid such as air orother compressible gas, or even incompressible fluids may also be usedwith the valve 28 to selectively move the piston 42 along the passageway39. However, the use of such gases or fluids, if incompatible with theefficient operations HVAC&R system 10, may need to remain separate fromthe refrigerant in the HVAC&R system 10. Alternately, piston 42 may alsobe magnetically or electrically displaced within the passageway 39.

For example, referring to FIG. 3, which is otherwise the same as FIG. 2except as shown, pressurized fluids are selectively provided to or bledfrom the pressure line 36 of a resonator 126 by the valve 28 from anindependent pressurized fluid source 154 and an independent fluidreceptacle 152. The fluid receptacle 152 receives pressurized fluid thatis bled from the pressure line 36 when the control panel 16 determinesthat the variable resonator length 50 needs to be increased to likewiseurge the piston 42 to move along the passageway 39 of the resonator body38 toward the cap 40. Conversely, the pressurized fluid source 154provides pressurized fluid to the pressure line 36 when the controlpanel 16 determines that the variable resonator length 50 needs to bedecreased to likewise urge the piston 42 to move along the passageway 39of the resonator body 38 toward the stop 48. The pressurized fluidsource 154 is pressurized to a higher level than the receptacle 152, andmay be pressurized to a much higher pressure than the discharge pressurefrom the discharge line 30, if desired. By providing highly pressurizedfluid in pressurized fluid source 154, it may be possible to eliminatethe need for the resilient device spring 46, and decrease the responsetime for movement of the piston 42 along the passageway 39 of theresonator body 38.

Optionally, a second valve 129 that is also controlled by the controlpanel 16 is provided to selectively provide pressurized fluid from ahigh pressure line 158 to a variable pressure line 156. To decrease theamount of pressure in the variable pressure line 156, pressurized fluidmay be bled to the receptacle 152 through a low pressure line 160. Toensure the pressurized fluid that is provided to the resonator body 38from the second valve 129 remains separate from the refrigerant fluid ofthe HVAC&R system 10, i.e., the refrigerant gas flowing in the dischargeline 30, the pressurized fluid from the second valve 129 is received inan expandable/contractable toroidal container 162. The container 162 isdisposed between the stop 48 and the piston 42 and is composed of aresilient material that permits the container 162 to expand or contractin response to sufficient differences between forces F_(CAP) andF_(STOP) which result in movement of the piston 42 with respect to thecontainer 162. Preferably, the container 162 remains adjacent to theinside the surface 41 of the resonator body 38 and collapses upon itselfas the container 162 is maintained in contact with the piston 42 as itmoves along the passageway 39, possibly by use of baffles. Optionally, aresilient device (not shown) may be provided inside the container 162.Additionally, the container 162 preferably provides a uniform crosssectional profile within the passageway 39 to further providepredictable noise attenuation behavior as the resonator length 50 isvaried as previously described.

It is appreciated that any number of combinations of independentlyprovided pressurized fluids from pressurized sources or receptacles maybe used with the pressurized refrigerant gas from the HVAC&R system, andthat these pressurized fluids may or may not need to be separated fromeach other. It is also appreciated that any number of valves may be usedto control the flow of the pressurized fluids as discussed.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A resonator arrangement for a closed fluid system comprising: a bodyhaving a passageway in fluid communication with a closed fluid system; apiston movable within the passageway wherein a position of the pistonwithin the passageway defines a noise attenuation frequency for theclosed fluid system; a control arrangement to selectively position thepiston within the passageway to generate a noise attenuation frequencycorresponding to a noise frequency generated by the closed fluid system.2. The resonator of claim 1 wherein the control arrangement includes apressurized fluid to selectively position the piston.
 3. The resonatorof claim 2 wherein the pressurized fluid to selectively position thepiston is taken from a pressurized fluid source that is independent ofthe closed fluid system.
 4. The resonator of claim 3 wherein thepressurized fluid to selectively position the piston is separated fromthe fluid in the closed fluid system.
 5. The resonator of claim 3wherein the pressurized fluid to selectively position the piston is thesame as the fluid in the closed fluid system.
 6. The resonator of claim2 wherein the pressurized fluid to selectively position the piston istaken from the closed fluid system.
 7. The resonator of claim 3 whereinthe pressurized fluid provided to selectively position the piston withinthe passageway in a direction to generate a decreased noise attenuationfrequency corresponding to the noise frequency generated by the closedfluid system is taken from a pressurized fluid source that isindependent of the closed fluid system; and wherein the pressurizedfluid provided to selectively position the piston within the passagewayin a direction to generate an increased noise attenuation frequencycorresponding to the noise frequency generated by the closed fluidsystem is taken from the closed fluid system.
 8. The resonator of claim7 wherein the pressurized fluid taken from the pressurized fluid sourcethat is independent of the closed fluid system is separated from thefluid in the closed fluid system.
 9. The resonator of claim 3 whereinthe pressurized fluid provided to selectively move the piston within thepassageway in a direction to generate an increased noise attenuationfrequency corresponding to the noise frequency generated by the closedfluid system is taken from a pressurized fluid source that isindependent of the closed fluid system; and wherein the pressurizedfluid provided to selectively move the piston within the passageway in adirection to generate a decreased noise attenuation frequencycorresponding to the noise frequency generated by the closed fluidsystem is taken from the closed fluid system.
 10. The resonator of claim9 wherein the pressurized fluid taken from the pressurized fluid sourcethat is independent of the closed fluid system is separated from thefluid in the closed fluid system.
 11. The resonator of claim 1 whereinthe closed fluid system is an HVAC&R system.
 12. The resonator of claim11 wherein the HVAC&R system includes a variable speed drive.
 13. Theresonator of claim 1 wherein the control arrangement includes a valve.14. The resonator of claim 1 wherein the control arrangement includes aproportional valve.
 15. The resonator of claim 1 further comprising aresilient device within the passageway to urge the piston to move in adirection within the passageway.
 16. The resonator of claim 15 whereinthe resilient device is a spring.
 17. The resonator of claim 15 furthercomprising a resilient device within the passageway to urge the pistonto move in a direction within the passageway toward the closed fluidsystem.
 18. The resonator of claim 15 further comprising a resilientdevice within the passageway to urge the piston to move in a directionwithin the passageway away from the closed fluid system.
 19. A variablenoise attenuation device for use with an HVAC&R fluid system comprising:a body having a passageway in fluid communication with an HVAC&R fluidsystem; a piston movable within the passageway wherein a position of thepiston within the passageway defining a noise attenuation frequency forthe HFAC&R fluid system; a proportional valve selectively providing apressurized fluid from the HVAC&R fluid system to the piston toselectively position the piston within the passageway to generate anoise attenuation frequency corresponding to a noise frequency generatedby the HFAC&R fluid system.
 20. The variable noise attenuation device ofclaim 19 wherein the proportional valve is in fluid communication with apositive displacement compressor.
 21. The variable noise attenuationdevice of claim 20 wherein the positive displacement compressor is ascrew compressor or a reciprocating compressor.
 22. The variable noiseattenuation device of claim 19 wherein the HVAC&R fluid system is aclosed HVAC&R fluid system.
 23. The variable noise attenuation device ofclaim 19 wherein the HVAC&R fluid system includes a variable speeddrive.
 24. A method for attenuating noise in a closed fluid system, thesteps comprising: providing a body having a passageway in fluidcommunication with a closed fluid system; providing a piston movablewithin the passageway wherein a position of the piston within thepassageway defines a noise attenuation frequency for the closed fluidsystem; providing a control arrangement for providing a pressurizedfluid to the piston; and positioning selectively the piston within thepassageway to generate a noise attenuation frequency corresponding to anoise frequency generated by the closed fluid system.
 25. The method ofclaim 24 wherein the step of providing the control arrangement forproviding the pressurized fluid to the piston includes providing thepressurized fluid from a pressurized fluid source that is independent ofthe closed fluid system.
 26. The method of claim 25 wherein the step ofproviding the control arrangement for providing the pressurized fluidfrom the pressurized fluid source that is independent of the closedfluid system further includes the step of separating the pressurizedfluid from the pressurized fluid source that is independent of theclosed fluid system from the fluid in the closed fluid system.
 27. Themethod of claim 25 wherein the pressurized fluid from the pressurizedfluid source that is independent of the closed fluid system is the sameas the fluid in the closed fluid system.
 28. The method of claim 24wherein the step of providing the control arrangement for providing thepressurized fluid to the piston includes providing the pressurized fluidtaken from the closed fluid system.
 29. The method of claim 25 whereinthe step of providing the control arrangement for providing thepressurized fluid from the pressurized fluid source that is independentof the closed fluid system to the piston includes providing thepressurized fluid from the pressurized fluid source that is independentof the closed fluid system to selectively position the piston within thepassageway in a direction to generate a decreased noise attenuationfrequency corresponding to the noise frequency generated by the closedfluid system; and providing the control arrangement for providing thepressurized fluid from the closed fluid system to selectively positionthe piston within the passageway in a direction to generate an increasednoise attenuation frequency corresponding to the noise frequencygenerated by the closed fluid system.
 30. The method of claim 29 whereinthe pressurized fluid from the pressurized fluid source that isindependent of the closed fluid system to selectively position thepiston within the passageway in a direction to generate the decreasednoise attenuation frequency corresponding to the noise frequencygenerated by the closed fluid system is separated from the fluid in theclosed fluid system.
 31. The method of claim 25 wherein the step ofproviding the control arrangement for providing the pressurized fluidfrom the pressurized fluid source that is independent of the closedfluid system to the piston includes providing the pressurized fluid fromthe pressurized fluid source that is independent of the closed fluidsystem to selectively position the piston within the passageway in adirection to generate an increased noise attenuation frequencycorresponding to the noise frequency generated by the closed fluidsystem; and providing the pressurized fluid from the closed fluid systemto selectively position the piston within the passageway in a directionto generate a decreased noise attenuation frequency corresponding to thenoise frequency generated by the closed fluid system.
 32. The method ofclaim 31 wherein the pressurized fluid from the pressurized fluid sourcethat is independent of the closed fluid system to selectively positionthe piston within the passageway in a direction to generate an increasednoise attenuation frequency corresponding to the noise frequencygenerated by the closed fluid system is separated from the fluid in theclosed fluid system.
 33. The method of claim 24 wherein the closed fluidsystem is an HVAC&R system.
 34. The method of claim 33 wherein theHVAC&R system includes a variable speed drive.
 35. The method of claim24 wherein the step of providing the control arrangement includes a stepof providing a valve.
 36. The method of claim 24 wherein the step ofproviding the control arrangement includes providing a proportionalvalve.
 37. The method of claim 24 further including an additional step,before the step of providing a control arrangement for providing apressurized fluid from the closed fluid system to the piston, ofproviding an algorithm, the step of providing an algorithm furtherincluding the steps of reading a variable speed drive frequency output;and comparing the variable speed drive frequency output to the positionof the piston in the passageway; and wherein the step of positioningselectively the piston within the passageway to generate the noiseattenuation frequency corresponding to the noise frequency generated bythe closed fluid system includes positioning selectively the pistonwithin the passageway to generate the noise attenuation frequencycorresponding to the noise frequency generated by the closed fluidsystem in response to the algorithm.
 38. The method of claim 24 furtherincluding an additional step, before the step of providing a controlarrangement for providing pressurized fluid, of providing a resilientdevice within the passageway to urge the piston into movement within thepassageway toward the closed fluid system.
 39. The method of claim 38wherein the resilient device is a spring.